Solid secondary electrochemical cells consist of a solid electrolyte interposed between an anode and a cathode. Preferably, the solid electrolyte is a single-phase material consisting of a solid polymeric matrix, an inorganic ion salt, and an electrolyte solvent. Preferably, the cathode is a composite material composed of an intercalation compound, carbon, solvents and a solid polymeric matrix. The anode may be composed of metals or of an intercalating material. For many advantageous reasons, interest centers on lithium-based electrochemical cells and intercalating cathodes.
Solid electrochemical cells offer a number of advantages over electrochemical cells containing a liquid electrolyte, including light-weight and high-energy density. Solid electrolytes are prepared in thin layers which reduce cell resistance and allow large drains at low-current density. The solid electrolyte is usually composed of a polymeric matrix, a suitable salt and an electrolyte solvent which aids conductivity and acts as a plasticizer for the polymer. The polymerization is normally carried out in a curing step performed on a composition, including a polymer precursor, or prepolymer, which undergoes radiation or heat-induced cross-linking reactions to form the polymeric matrix. Suitable polymer precursors include, for example, poly(oxyalkylene) ethers, such as polyethylene oxide, and acryloyl-derivatized poly(oxyalkylene) ethers.
The solid electrolyte also contains a solvent chosen for lower volatility and excellent capability with the other components of the electrolyte as evidenced by conductivity, charge capacity and life-time of the cell. Suitable solvents well-known in the art for use in solid electrolytes including, by way of example, organic carbonates and glymes.
In a preferred method of manufacturing a solid electrochemical cell, the solid electrolyte is cured on the surface of a cathode. Typically, the cathode is itself prepared by coating a mixture of cathode-active material, the electroconductive such as carbon, a solid matrix forming polymer precursor, and an electrolyte solvent on a current collector followed by curing with an electron beam. If the solid electrolyte is then formed on this cathode surface, an anode material can then be laminated on to the solid electrolyte to form a solid electrochemical cell.
It has been suggested that a conductive polymer may be used to replace carbon as the conductive material in composite electrodes, particularly, composite cathodes. While the art recognizes that dopants in one form or another are a necessary compliment to non-ionic organic materials in order to function as efficient conductors, and that a means must be found to efficiently incorporate the conducting polymer and dopant into the composite electrode, means of satisfactorily accomplishing this end have been lacking.
Many conducting polymers are difficult to work with and some are simply intractable high molecular weight materials, insoluble in ordinary solvents and prone to decomposition below their melting or softening point.
Co-pending U.S. patent application Ser. No. 08/163,209, filed Dec. 6, 1993, the disclosure of which is incorporated herein in its entirety, disclosed the replacement of carbon as the conductive component of the cathode in an electrochemical cell. The goal of replacing carbon with conducting polymer was to improve the conductivity, in particular, the electronic conductivity of the cathode with a lightweight substitute material.
It was suggested that compatible cathode-active materials, i.e., intercalation compounds, be mixed with electronically conducting polymer, and a binder, such as a polymeric binder, to form a positive cathode plate under pressure. Suitable cathode-active materials included vanadium oxides, and suitable conductive polymers included polyaniline.
Electrodes made of a major amount of conducting polymer have been developed for solid electrochemical cells, i.e., Fiona M. Gray, "Solid Polymer Electrolytes", VCH Publishers, Inc., New York, 1991, pp. 5-9; U.S. Pat. Nos. 4,222,903; 4,519,939; 4,519,940; and 4,579,679, the disclosures of each of which is incorporated herein in its entirety.
It would be advantageous if the method of making composite electrodes of cathode-active material and conducting polymers could be improved. In particular, the art is searching for methods of handling polymeric conductors more efficiently to produce a more conductive, less resistive, lower impedance cathode of high charge capacity and cyclability.